1. Field of the Invention
The present invention relates to an internal combustion engine provided with double system of fuel injection including a port fuel injection (PFI) system and a direct injection (DI) system.
2. Related Art
In an internal combustion engine of an automobile, a knocking may be caused at a time when an automobile is rapidly accelerated from a low speed running thereof.
The knocking is a phenomenon in which unburned fuel-air mixture in front of a propagation flame is compressed to a high temperature and then self-ignited due to incompatibility between engine load and engine ignition timing, and a large pressure wave generated at a combustion period in a combustion chamber vibrates a wall of the combustion chamber. According to the generation of such knocking, temperature and pressure in the combustion chamber are abnormally increased, which may cause damage to the combustion chamber. Because of this reason, various counter-measures have been considered in prior art, for example, as disclosed in Japanese Patent Laid-open (KOKAI) Publication No. HEI 10-141194).
In this publication, there is disclosed a control unit for controlling an ignition timing of an internal combustion engine, in which the ignition timing is set to a maximum torque generation timing (MBT: Minimum Spark Advance for Best Torque) in a low load range at which any knocking never be caused, and on the contrary, the ignition timing is set to a knocking limit torque generation timing (TK) based on knocking detection in a high load range at which the knocking may be liable to be caused. The knocking limit means a timing at which a knocking noise (trace knock) is first detected during advancing of the ignition timing.
However, in the invention disclosed in the above prior art publication, the ignition timing control is applicable to an internal combustion engine in which one fuel supply device is used for one cylinder.
Incidentally, fuel injection modes is classified into a PFI system (Port Fuel Injection System) in which fuel is injected from an injector to an intake pipe for each cylinder and a DI system (Direct Injection System) in which fuel is injected directly in each cylinder from an injector.
In the PFI system, fuel is easily mixed uniformly with air, and accordingly, in low revolution speed operation of the engine (low engine revolution speed), the engine performance can be fully achieved. On the other hand, in the DI system, since a lot of air is sucked into the cylinder, the high engine performance can be achieved in high revolution speed operation of the engine (high engine revolution speed). In order to achieve more effective combustion performance of the internal combustion engine by utilizing the above advantages of both systems, there may provide a system in which both PFI system and DI system are parallelly arranged for each cylinder of the engine so that a ratio of fuel injection quantity from each of both the systems (i.e. injectors) is changed in accordance with the engine revolution and engine load.
However, the PFI system and DI system differ from each other in their maximum torque generation timings, the knocking limit torque generation timing and so on. Therefore, even if the invention of the above-mentioned prior art patent publication is applied to the case in which both systems are parallelly arranged for each cylinder of the engine, an optimum ignition timing cannot be achieved.